This application proposes to generate embryonic stem (ES) cell lines from Ts65Dn mice, a mouse model for Down syndrome (DS). Down syndrome, trisomy of human chromosome 21, is the most common cause of mental retardation. The widest used mouse model is the Ts65Dn mouse. The Ts65Dn mouse strain has a translocation of part of mouse chromosome 16 to chromosome 17, making the mouse trisomic for a large fraction of genes syntenic to human chromosome 21. Most importantly, the Ts65Dn mouse shows a large array of phenotypes found in DS patients, specifically cognitive deficits. Pluripotent ES cell lines containing the Ts65Dn translocation would be highly valuable research tools for a number of reasons. First, a Ts65Dn ES cell line would enable researchers to study in detail, in vitro, events during development and differentiation of specific cell lineages, such as hematopoietic, muscle, and neuronal cells. Second, Ts65Dn ES cell lines will allow one to genetically alter the trisomic state of individual genes or combinations of genes through gene targeting in vitro. These genetically altered ES cells can then be compared in vitro to the fully trisomic Ts65Dn line. Thirdly, genetically altered Ts65Dn ES cell lines can be used to establish new mouse lines with defined genetic alterations of the fully trisomic Ts65Dn mouse strain. The analysis of Ts65Dn ES cell derivatives and mouse derivatives with individual genes or combinations of genes subtracted from the trisomic set will allow conclusions about the role of individual genes in the pathogenesis of specific aspects of Down syndrome. The investigator proposes to derive ES cell lines from Ts65Dn mice and to demonstrate the general capability of Ts65Dn ES cell lines to develop in vitro and in vivo. Specific Aim 1 will derive Ts65Dn-ES cell lines from blastocysts of Ts65Dn females. Specific Aim 2 will assess the effects of the trisomic set of genes on in vitro differentiation of ES cells. Specific Aim 3 will test the potential of Ts65Dn ES cells for in vivo development. Generating Ts65Dn ES cell lines will provide an important research tool for a large number of investigators with a wide array of applications, specifically for research in neural development, the biology of cognition, and mental retardation and their specific applications to Down syndrome.